In North American freight railroad service, conventional three-piece freight car trucks, having two wheelsets, have evolved to satisfy a variety of important operating and economic requirements. Among other requirements, they must be capable of safely supporting, and equalizing very high wheel loads over a wide range of track conditions while delivering a high level of economic value to the railroads that use them. In addition to those basic criteria, the trucks and their parts must be interchangeable throughout the system of interconnected railroad networks. The three-piece trucks in service today have, to a large extent, met these requirements, because their general designs are simple, flexible, durable, and reliable. However, in this evolutionary process, a major aspect of truck design for performance efficiency has been largely ignored, design for warp friction moment.
When a conventional three-piece truck encounters sufficient energy in the course of its normal use, usually due to high-speed operation, the wheelsets are forced to move laterally relative to the track and relative to one another causing the instability known as "truck hunting". Truck hunting is undesirable, because it causes high lateral forces to be imparted to the rail vehicle and its lading, and because it produces increased drag on the locomotive, resulting in reduced efficiency. Likewise, when a conventional three-piece truck encounters a curve in the normal course of its use, the wheelsets are often forced to move laterally relative to one another resulting in a condition known as "truck warp". Truck warp is undesirable, because it causes a high angle of attack to arise between the leading wheelset and the rail, resulting in high rates of wear on the rails and wheels. Whether they are a result of high speed or curving, truck hunting and truck warp are generally characterized by a lateral displacement of the wheelsets relative to one another, and a change of the square relationship of the side frames relative to the bolster into an angular relationship.
Testing of conventional three-piece freight car trucks involved in heavy axle load derailments has shown that a large proportion of the interaxle shear stiffness that governs their performance is attributable to the side frame to bolster connection. However, current designs of this connection have an inherent problem in that they only provide resistance to unsquaring movements between the side frames and bolster up to the limit of the coulomb friction force that binds these connections. Recent theoretical modeling, and laboratory testing have confirmed that the warp friction moment is the critical determining factor in the performance of the three-piece truck.
The side frame to bolster connection design of three-piece trucks is generally characterized by a right triangle shaped friction wedge in contact with and contained by a pocket in the bolster on one side, a vertical surface of the side frame on another, and a spring on the third side. The connection is comprised of three load bearing interfaces: the Spring Seat Surface, the Slope Surface, and the Column Surface. The wedge surfaces are oriented in the shape of a right triangle with the spring seat and column surface oriented at a right angle to each other, and the slope surface oriented at an acute angle to the column surface. The wedge is oriented with the column surface vertically to allow sliding motion of the bolster relative to the side frame due to dynamic forces of the rail vehicle body. The wedge slope surface bears on the bolster pocket slope surface, which acts to direct the force of the spring from the spring seat surface into the column surface. As a result of the wedge configuration and orientation, a force balance is formed on the friction wedge, at the three interfaces, that is governed by the relative position and movement of the bolster to the side frame.
Three different force balances are possible: the spring Compression Stroke force balance, the spring Decompression Stroke force balance, and the truck Warp Action force balance. The compression and decompression stroke force balances are the force balances that describe the coulomb damping forces in the three-piece truck, and they have been used for many years by design engineers to design friction wedges for vertical damping. These two force balances are governed by the wedge angle, the spring force, and the coefficients of friction between the materials of the wedge and the column and slope surfaces respectively. The warp action force balance describes the forces that act on the wedge under interaxle shear force conditions, and it gets its name from the interaxle shear or truck warp forces that generate the wedge forces. Under warp action, the friction forces that otherwise act in opposite directions, act upward in the same direction, and bind the wedge between the column and side frame up to the limit of the static friction forces at those interfaces.
The warp action force balance that describes the warp action forces on the wedge is new, and has neither been described in the prior art nor publication literature. It was discovered through a parameter effect analysis of the wedge force balance parameters. The objective of the analysis was to determine the effect on the damping force of the governing parameters: wedge angle, friction coefficient, and spring force. The analysis revealed the exponential nature of the damping force to the wedge angle and friction coefficient. The association of this fact with the fact discovered in the derailment investigations that trucks with smaller wedge angles were much less likely to derail, lead to the discovery that a unique frictional force balance on the wedge must exist under truck warp force conditions.
The expanded parameter analysis revealed the same type of exponential relationship of the warp friction moment to the wedge angle and friction coefficient as the damping force analysis did. This lead to the discovery that, although both the damping force and the warp friction force increased exponentially with decreasing wedge angle and increasing friction coefficient, the warp friction force increased much more rapidly than the damping force. This fact implied the probable existence of a wedge angle and spring force combination that, given a certain friction coefficient, would produce a wedge design with a high warp friction moment and a low damping force.
The probable existence of an "optimum" combination of the essential wedge force balance parameters lead to the development of a model designed to determine the values of the parameters by means of objective inputs. As a result, one object of the present invention is the math model so derived, and entitled, "Method for the Design of a Friction Wedge and Side Spring Optimized for Lateral Warp Friction Moment and Vertical Damping Force". The essence of the model is the warp action force balance combined with the truck warp force balance, in a set of simultaneous equations with the compression damping force balance.
The model uses the basic objective inputs of: wedge width, wedge friction coefficients, and damping ratios; and rail vehicle weights, major truck dimensions, center plate and side bearing friction coefficients, and rail friction coefficient. These inputs can be divided into two groups: one group that describes the friction wedge characteristics, and one group that describes the truck characteristics at the empty and loaded car conditions. Although all the parameters of both groups are defined objectively, one parameter from the wedge group and two parameters from the truck group require some discretion in setting their values in order to achieve the best possible optimized solution. The rail friction coefficient and the center plate (and side bearing) friction coefficient are the primary driving factors of the empty and loaded car warp forces respectively, and the damping ratio is the primary driving factor of the damping forces. Therefore, these three parameters are designed to be determined on the basis of the required level of warp resistance and damping force for the application of the truck.
With the basic input parameters determined, the model produces a solution in terms of the unknown friction wedge, and side spring dimensions: wedge angle, wedge height, wedge depth, and work point; and spring bar diameter, outer diameter, and free height respectively. Together with the inputs such as wedge width, and spring solid height, the model solution provides the exact dimensions for a complete friction wedge and side spring design optimized to produce a predetermined combination of warp friction moment and damping force. In addition to providing the dimensions for these designs, the model also provides an exact solution for the number and type of load springs necessary to design a complete suspension arrangement that is consistent with the wedge and side spring design solution.
As stated above, this model is designed to determine the optimum wedge and spring design solution for any combination of car load, truck size, and wedge material. The discretionary inputs are designed to allow the engineer the flexibility to adjust the input parameters to produce the wedge and spring design solution desired. However, the discretionary inputs are rooted in real terms that have objective definitions. Therefore, an optimum wedge and spring design solution can be found by applying objectively determined versions of the discretionary inputs. When this is done, and some allowance is made for the natural variation inherent in the input parameters, a pattern of wedge design emerges that has a very specific set of ranges of the essential design parameters.
Of all the essential wedge design parameters, the wedge angle is, by definition, the most essential, because it is the dimension that defines the triangular shape of the wedge and has the greatest controllable effect on the damping and warp friction forces. The range of wedge angle that emerges from the completely objective input case lies just below the typical angular range of friction wedge design. In combination with a sufficient wedge width, a moderate wedge friction coefficient, and a certain spring force, the smaller than normal wedge angle becomes a powerful feature for producing a combination of high warp friction moment with low to moderate damping force in one friction wedge and side spring design.
Given this fact, it is the object of this invention, in addition to the claims of the design method model, to claim two preferred embodiments of the friction wedge and spring designed to generally accepted values of the objective inputs described in this application. The two preferred embodiments are to be wedge and spring couples that are designed to the solutions determined by the design method model. The range of wedge and spring couple design is to be determined by generally accepted values of variation of the objective inputs to the model.